1. Field of the Invention
The present invention relates to an optical communication system using optical phase conjugation to suppress waveform distortion caused by chromatic dispersion or group velocity dispersion (GVD) and the optical Kerr effect and, more particularly, to the use of a polarization maintaining fiber in such an optical communication system and to the transmission of a wavelength division multiplexed (WDM) light signal in such an optical communication system.
2. Description of the Related Art
Optical communication systems typically transmit optical signals through a fiber (or "optical fiber") over long distances of several hundred to several thousand kilometers, and with transmission speeds as high as several Gb/sec to 10 Gb/sec or more. However, the transmission quality of the optical signals travelling through the fiber can significantly deteriorate due to GVD in the fiber. As a result, the transmission speed and/or transmission distance is usually limited by the effect of GVD.
Moreover, intensity modulation or amplitude modulation is used to produce a signal light having a waveform of optical pulses. However, the resulting waveform may be distorted for reasons other than GVD. For example, waveform distortion can be caused by GVD and the optical Kerr effect.
The following is a discussion of the relationship of GVD to the optical Kerr effect. Assume that an optical pulse propagates in a dispersive medium. When an unchirped pulse passes through a normal dispersion medium (.differential..sup.2.beta./.differential..omega..sup.2 &gt;0, where .beta. and .omega. denote the propagation constant and the angular frequency of the light, respectively), the pulse is shifted toward a lower frequency side at its rising edge and is shifted toward a higher frequency side at its falling edge. By contrast, in the case of an abnormal dispersion medium (.differential..sup.2.beta./.differential..omega..sup.2 &lt;0), the pulse is shifted toward a higher frequency side at its rising edge and is shifted toward a lower frequency side at its falling edge.
Further, in a normal dispersion medium, the longer the wavelength, the higher the group velocity. By contrast, in an anomalous dispersion medium, the shorter the wavelength, the higher the group velocity. Therefore, in either case, the pulse width is increased.
When the light intensity is great, the refractive index is changed by the following value due to the optical Kerr effect.
Equation (1): EQU .DELTA.n(t)=n.sub.2.vertline.E(t).vertline..sup.2
In the above Equation (1), n.sub.2 is the nonlinear refractive index. For example, with a silica fiber, the nonlinear refractive index n.sub.2 is approximately 3.2.times.10.sup.-20 m.sup.2 /W.
When an optical pulse is affected by the optical Kerr effect in a nonlinear medium, the spectrum is chirped as shown by the following Equation (2).
Equation (2): ##EQU1##
where .DELTA.z denotes the interaction length. This phenomenon is generally termed self-phase modulation (SPM).
Due to SPM, the optical pulse is shifted to a lower frequency side at its rising edge and is shifted toward a higher frequency side at its falling edge. Because of the chirping caused by such SPM, the influence of the dispersion is rendered more noticeable, thereby increasing the pulse distortion.
Therefore, when the optical pulse is affected by the optical Kerr effect in a normal dispersion medium, the pulse broadens more than in the case of dispersion alone. Moreover, in an anomalous dispersion medium, pulse compression occurs. An optical soliton is obtained by counter-balancing GVD and SPM.
In an anomalous dispersion medium, a high signal-to-noise ratio can be advantageously retained by applying pulse compression derived from SPM. However, recent research developments enable satisfactory transmission with high-level optical power by the use of an optical amplifier and a dispersion-shifted fiber (DSF) having a relatively small GVD. Therefore, it is uncertain whether pulse compression will provide improved results. In other words, large waveform distortion is generated as the pulse compression effect is rendered excessive. Particularly in the case of non-return to zero (NRZ) pulses, concentrative pulse compression occurs at rising and falling portions of the pulses, so that severe waveform changes are induced. In an extreme case, a falling portion passes a rising portion to eventually cause a phenomenon that one pulse is split into several pulses.
In view of the above, significant waveform distortion occurs from GVD and the optical Kerr effect. Such waveform distortion can be reduced by using conventional fibers having approximately zero dispersion at wavelength regions of 1.3 .mu.m and 1.55 .mu.m. However, without other countermeasures and due to GVD and the optical Kerr effect, such fibers may not adequately suppress waveform distortion.